Mental retardation from PKU has essentially been eliminated due to routine newborn screening and early dietary treatment of this disease. However, there is a high frequency of neuropsychological and emotional deficits among the early treated children. There is a growing body of evidence that many of these difficulties result from deficits in higher function, so called executive function. Since executive function seems to be controlled by the prefrontal cortex, and since the prefrontal cortex has one of the highest levels of dopamine turnover in the brain, thus making it especially sensitive to reductions in dopamine, and since dopamine is a metabolite of tyrosine, it has been hypothesized that the executive function deficits result from insufficient brain tyrosine. The objective of this study is to determine the blood tyrosine response to L-tyrosine supplementation in children and young adults with phenylketonuria (PKU) who are on phenylalanine restricted dietary treatment. Should it be found that the larger than usual amount of L-tyrosine supplementation proposed to be given does produce a marked and sustained blood tyrosine response without markedly increasing the accumulation of tyrosine metabolites, this treatment, which is much less expensive than 2-(2-nitro-4-fluoromethylbenzoyl)-1,3 cyclohexanedione(NTBC), could be considered.